The present invention relates to a method for forming reception beams, an apparatus for forming reception beams and a matched filter for forming a reception beam from received short-pulse signals in a sonar or an ultrasonic diagnostic apparatus.
Apparatuses for forming reception beams by using a receiving array formed of a plurality of ultrasonic transducer elements have conventionally been available for practical use. These Apparatuses include, for example, a time-domain beamformer comprising delay circuits which are individually connected to the corresponding transducer elements, and a frequency-domain beamformer comprising phase-shift circuits which are individually connected to the corresponding transducer elements. Although it is possible to form a receiving beam suited for the reception of short-pulse signals by using the conventional apparatuses, these apparatuses are only used in limited systems including those for military applications that utilize relatively low frequencies, because the conventional beamforming apparatuses require a large-scale circuit configuration.
In sonar systems generally used in private sectors, on the other hand, it is practically impossible to provide delay circuits connected to individual transducer elements of an array due to limitations in physical size and product cost. Thus, the commercial sonar system sequentially samples signals received by the individual transducer elements at specific time intervals and forms a receiving beam by using sample data thus obtained. Since it is impossible to continuously monitor all the signals picked up by the individual transducer elements, the receiving beam is formed on the assumption that pulselength is long enough to ensure that a pulse of incoming ultrasonic waves would uniformly cover the entire receiving transducer array (or at least all the transducer elements that are used for forming the receiving beam). An analog phase shifter method, a complex discrete Fourier transform (DFT) method and a matched filter method are known examples of this type of beamforming.
In a system whose design is based on the assumption that a pulse of incoming ultrasonic waves would uniformly cover the entire receiving transducer array, however, beamforming performance would considerably deteriorate when the pulselength of a signal of incoming ultrasonic waves decreases and the incoming waves are not received simultaneously by the transducer elements used for forming the receiving beam.
In a bottom detecting sonar, on the other hand, it is necessary to shorten the pulselength of the incoming ultrasonic signals (return echoes) by shortening pulselength of a transmission signal to detect the sea bottom with high accuracy. In the commercial sonar systems and ultrasonic diagnostic apparatus, there is a growing tendency today to use shorter pulses or pulse compression by frequency modulation to achieve high resolution. Under these circumstances, the aforementioned conventional receiving beam-forming methods would pose problems related to performance deterioration.
There will be explained hereinafter How short-pulse waves arrive at a generally used receiving array referring to FIGS. 19 and 20.
FIG. 19 is a schematic diagram of a generally cylindrical receiving ultrasonic transducer array. The generally cylindrical shape of this receiving array has a radius of 125 mm with a sectorial portion of the cylindrical shape cut away, leaving a sectorial portion whose a central angle is 238.5°, for example. 160 transducer elements are arranged at 1.5° intervals on the 238.5° sectorial portion (1.5×159=238.5°). A receiving circuit for processing echo signals picked up by this receiving array forms a receiving beam using 60 transducer elements contained in about a 90° sector. The receiving array steers this receiving beam or, in other words, forms 101 receiving beams at 1.5° angular intervals, to scan a 150° sector area.
Provided that the frequency of return echoes is 320 kHz, the distance between the most frontal transducer elements (closest to the current beam direction) and the rearmost transducer elements simultaneously used for forming the receiving beam is equivalent to about 7.5 times the wavelength. If a return echo is a short-pulse signal having a carrier frequency whose pulselength is equal to 6 times the wavelength of the carrier, for example, the return echo can not be received at the same time by the 60 transducer elements contained in the 90° sector that are used for forming the receiving beam in the current beam direction as shown in FIG. 19.
FIG. 20 shows an example of a linear ultrasonic transducer array formed by arranging a plurality of transducer elements in a straight line. More specifically, 80 transducer elements are arranged at intervals equal to half the wavelength of the carrier included in incoming signals in this linear array. If waves of return echoes arrive from a direction almost perpendicular to the length of the array as illustrated by solid lines in FIG. 20, the return echoes can be simultaneously received by the entire array even when the return echoes are short-pulse signals having a carrier frequency whose pulselength is equal to 6 times the wavelength of the carrier, for example. If, however, the same short-pulse waves arrive at an incident angle of −60° as illustrated by broken lines in FIG. 20, only a limited number of transducer elements of the linear array can simultaneously receive the return echoes, resulting in a significant deterioration in beamforming performance, such as widening of beam angle and a decrease in sensitivity.